Cationic Surface Segregation in Donor-Doped SrTiO3 Under Oxidizing Conditions

Abstract

The influence of high temperature oxygen annealing on (100) oriented donor-doped SrTiO3 single crystals was studied. Crystalline precipitates were found on the optical scale on surfaces of lanthanum-doped as well as niobium-doped specimens with donor concentrations above 0.5 at.%. The amount of the secondary phase increases with the doping level, oxidation temperature and oxidation time. EDX analyses of the crystallites reveal a SrOx composition. The formation of the observed secondary phase is discussed by means of the defect re-equilibration of the cation sub-lattice. In view of the point defect model for donor-doped perovskites, n-conducting SrTiO3 changes its compensation mechanism during an oxidation treatment from “electronic compensation” (N D = n) to “self-compensation” (N D = 2[V Sr″]) by forming cation vacancies. Due to the favored Schottky-type disorder in perovskites, the formation of strontium vacancies is accompanied by a release of strontium from the regular lattice. Since the excess strontium is found to be situated at the surface in form of SrO-rich precipitates only, we propose the formation of strontium vacancies via a surface defect reaction and the chemical diffusion of strontium vacancies from the surface into the crystal as the most probable re-equilibration mechanism for the oxidation treatment of single crystals. The introduced mechanism is in contrast to an established model which proposes the formation of Ruddlesden-Popper intergrowth phases SrO·(SrTiO3)n in the interior of the crystal.